Function alloc_str could be capacity overflow

[cicilzx]

Hi, I'm developing a fuzzer to test the safe abstraction, and I fonnd a segmentation fault when calling function src/lib/alloc_str().

This is a test case:

#[allow(warnings, unused)]
#[test]
fn test_alloc_str_jsItp() {
    let len: usize = 10830581488692310795;
    let _ = alloc_str(len);
}

Also, because it's a public API, if call this API directly like this:

use triple_accel;

fn main() {
    let len: usize = 10830581488692310795;
    let _ = triple_accel::alloc_str(len);
}

The output is as follows:

---- test_alloc_str_jsItp stdout ----
len = 10830581488692310795
thread 'test_alloc_str_jsItp' panicked at library/alloc/src/raw_vec.rs:25:5:
capacity overflow
stack backtrace:
   0: rust_begin_unwind
             at /rustc/d84b9037541f45dc2c52a41d723265af211c0497/library/std/src/panicking.rs:652:5
   1: core::panicking::panic_fmt
             at /rustc/d84b9037541f45dc2c52a41d723265af211c0497/library/core/src/panicking.rs:72:14
   2: alloc::raw_vec::capacity_overflow
             at /rustc/d84b9037541f45dc2c52a41d723265af211c0497/library/alloc/src/raw_vec.rs:25:5
   3: alloc::raw_vec::handle_error
             at /rustc/d84b9037541f45dc2c52a41d723265af211c0497/library/alloc/src/raw_vec.rs:593:29
   4: alloc::raw_vec::RawVec<T,A>::with_capacity_zeroed_in
             at /rustc/d84b9037541f45dc2c52a41d723265af211c0497/library/alloc/src/raw_vec.rs:178:25
   5: <T as alloc::vec::spec_from_elem::SpecFromElem>::from_elem
             at /rustc/d84b9037541f45dc2c52a41d723265af211c0497/library/alloc/src/vec/spec_from_elem.rs:25:31
   6: alloc::vec::from_elem
             at /rustc/d84b9037541f45dc2c52a41d723265af211c0497/library/alloc/src/vec/mod.rs:2739:5
   7: triple_accel::alloc_str
             at ./src/lib.rs:199:17
   8: triple_accel::test_alloc_str_jsItp
             at ./src/lib.rs:262:13
   9: triple_accel::test_alloc_str_jsItp::{{closure}}
             at ./src/lib.rs:247:26
  10: core::ops::function::FnOnce::call_once
             at /rustc/d84b9037541f45dc2c52a41d723265af211c0497/library/core/src/ops/function.rs:250:5
  11: core::ops::function::FnOnce::call_once
             at /rustc/d84b9037541f45dc2c52a41d723265af211c0497/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.